What is radiocarbon dating based on?

Radiocarbon dating is based on the fact that organisms contain approximately equal amounts of normal 12C and 14C (carbon-12 and carbon-14). Carbon 14 is radioactive, so it decays over time into other atoms. When an organism dies, it stops assimilating more carbon, so the 14C is no longer being replaced. Thus it decays until it is eventually gone. Within in about thirty-thousand years, however, the amount of 14C that is left can be used to calculate about when the organism died based on the fact that all radioactive decay occurs with a given half life. The half-life of a radioactive material is the amount of time that is required for half of the substance to decay. Each material has a unique half life which remains constant until there is very little of the sample left.